Audio system



y 1964 o. a. JUVONEN 3,142,724

' AUDIO SYSTEM Filed Feb. 8, 1962 AAAAAAAAAA 'VY'" "1' INVENTOR.

ONNI G. JUVONEN me W ATTYS.

United States Patent "cc 3,142,724 AUDIO SYSTEM Onni G. Juvonen, Chicago, Ill., assignor to Motorola, Inc, Chicago, 131., a corporation of Illinois Filed Feb. 8, 1962, Ser. No. 171,871 6 Claims. (Cl. 1791) This invention relates generally to audio systems and in particular to an audio output circuit including a transistor and a loudspeaker having a winding directly connected to the transistor.

It is desired in many applications to use inexpensive audio systems including a transistor amplifier and a loudspeaker coupled thereto. It is common practice to couple the loudspeaker to the transistor through a transformer which provides the required impedance matching, and which isolates the loudspeaker from the direct current bias circuit of the transistor. However, a transformer is an expensive component, and to reduce the cost it is desired to eliminate this component. It has been proposed to directly couple the loudspeaker to a push-pull transistor output circuit wherein the direct current from the two transistors balance each other and produces no effect on the loud speaker. However, it has not been possible to connect the loudspeaker directly to the tran sistor in a single ended circuit because the direct current bias produces an objectionable off-centering effect on the loudspeaker.

It is therefore an object of the present invention to provide a single ended audio output stage including a transistor and a loudspeaker having a winding directly connected to the transistor.

Another object of the invention is to provide a loud speaker having its winding directly connected to a transistor, with a compensating direct current applied to the winding to neutralize the eifect of the transistor biasing current.

A further object of the invention is to provide an audio stage having a loudspeaker directly connected to a transistor, and which may be modified to permit the connection of a plurality of loudspeakers.

A feature of the invention is the provision of an audio output circuit including a transistor amplifier and a loudspeaker with a winding having a grounded intermediate tap and having one end terminal connected to the. transistor and completing the bias circuit therefor, with the other end terminal connected through a resistance to the bias source to balance out the field produced by the transistor bias current. The resistance may be provided by the pilot lamp of a receiver or amplifier so that the power consumption is held to a minimum.

Another feature of the invention is the provision of an audio output stage for an automobile radio receiver including a common emitter transistor output amplifier and a loudspeaker having a tapped winding forming two sections, with bias current applied to the transistor from the automobile electrical system through a circuit completed from the collector electrode of the transistor to ground through one section of the winding, and the other winding section being connected between the electrical system of the automobile and ground so that the direct current in the two loudspeaker winding sections oppose and the loudspeaker diaphragm is held in a central position. The noise pulses appearing in the automobile electrical system are also balanced out in the winding sections.

A further feature of the invention is the provision of an audio output stage as described in the preceding paragraphs wherein the three connections to the loudspeaker may be broken and a choke or transformer and a resistor in series therewith are connected to complete the transistor output circuit to provide optimum gain, and

3,142,724 Patented July 28, 1964 wherein a plurality of loudspeakers are coupled tothe choke or transformer through a fader control so that the audio signals can be divided between the speakers in a desired manner.

The invention is illustrated in the accompanying drawing wherein:

FIG. 1 illustrates the audio circuit in accordance with the invention; and

FIGS. 2 and 3 illustrate modifications of the circuit which may be used for connecting a plurality of loudspeakers.

In practicing the invention there is provided an audio output stage including a transistor amplifier which is energized from a source of direct current potential. The stage may be used in an automobile radio receiver in which case it will be energized from the electrical system of the automobile. A loudspeaker is provided which may be of the electrodynamic type having a voice coil which moves in an air gap in a magnetic field. The voice coil winding has an intermediate tap which is grounded, and one end of the winding is connected to the transistor so that the output current flows through one section of the winding. The impedance of this section of the winding may be matched to the transistor to provide optimum output. The other end terminal of the loudspeaker winding is connected to the potential supply to provide direct current therein which opposes the direct current in the first section. Therefore, the direct current fields produced by the winding sections are neutralized and this will permit the voice coil to be positioned in its normal or central position in the air gap. The second section of the loudspeaker winding may be connected through a resistance which is selected to balance the fields. This resistance may be formed by the pilot lamp of the amplifier, or of a receiver in which it is used, to thereby minimize the power requirements. The transistor may be connected in a common emitter circuit with the input signal being applied between the base and emitter electrodes and the output signal derived between the emitter and collector. In such case the collector is connected to one end terminal of the loudspeaker and bias potentials are applied to the base and emitter electrodes.

In order to connect a pair of loudspeakers to the amplifier, as is required for front and rear seat speakers of an automobile receiver, the three connections to the loudspeaker Winding may be broken and a series circuit including a transformer or choke coil and a resistor in series are connected thereto. The transformer or coil may be selected to provide optimum audio output. The loudspeakers may be connected through a fader potentiometer to control the relative levels of the signals applied thereto, to thereby control thte audio outputs therefrom.

Referring now to the drawings, in FIG. 1 there is shown the audio output stage of the invention to which input signals are applied through transformer 10. The input signals may be derived from a preceding stage in a radio receiver, or from a preamplifier or any other audio source. The signals are applied from the secondary of transformer ill between the base electrode 12 and the emitter electrode 13 of the transistor 11. The circuit from the secondary winding of transformer 10 is completed through resistor 15. Bias potentials are applied to the transistor from potential source 16 which may have a voltage of the order of 13 volts. This voltage is used as an example since this is the voltage available from the electrical system of an automobile which includes a battery having a nominal voltage of 12 volts. This potential is applied through resistor 17 to the emitter electrode, and a portion of the potential is derived from the voltage divider formed by resistors 18 snearaa and 19 and applied through the secondary of transformer to the base electrode 12.

A loudspeaker 20 is connected to the transistor 11, and may be of the electrodynamic type having a permanent magnet field and a voice coil 21 movable within the air gap of the magnetic field. The voice coil or loudspeaker winding 21 includes end terminals 22 and 23 and a center terminal 24. Accordingly, a first winding section 21a is provided between the terminals 22 and 24 and a second winding section 21b is provided between the terminals 23 and 24. The end terminal 22 is connected to the collector electrode 14 of the transistor and the end terminal 23 is connected through pilot lamp 25 to the terminal In which provides a positive potential. The center terminal 24 of the loudspeaker winding is connected to reference potential or ground.

The transistor 11 is biased to be conducting so that direct current flows through the output circuit from terminal 16 through resistor 17 to the emitter electrode 13 of transistor 11 and from the collector electrode 7.4 to terminal 22 and through the winding section 21a to ground. When the audio signals are applied these signals will be amplified in the transistor and will likewise appear in the winding section 21a. The direct current through the transistor will cause a bias which will tend to hold the winding 21 offset from its normal position in the air gap of the loudspeaker magnetic structure. To overcome this the direct current through the pilot light 25 and through the second winding section 21b produces an opposing field which will tend to center the loudspeaker winding. The winding section 21:: is constructed to have an impedance to provide an optimum match with the transistor 11 to provide maximum overall gain. The impedance of the winding section 21b is not critical and the desired field therein may be provided by controlling the current therethrough, which is in turn controlled by the resistance connected in series therewith. This is provided by the pilot lamp 25. In the event that a pilot lamp of the desired resistance is not available, resistance can be connected in series or parallel therewith to provide the desired current. In certain applications it may not be desired to use a pilot lamp and another resistor device can be substituted therefor.

Although some audio power is lost through the winding section 21b, it has been found that by proper design of the winding the power loss will be less than five percent. This loss is more than overcome by the reduced cost resulting from the elimination of the output transformer normally provided to couple the loudspeaker to the transistor. Although the impedance of the winding section 21a is relatively critical, this can be determined by adjusting the position of the tap on winding 21 as the section 21b is not critical.

In addition to providing current to oppose the field produced by the direct current in the winding section 21a, the current through the winding section 21b will also balance out noise which may appear in the energizing potential at the terminal 16. In an automobile radio receiver, as previously mentioned, the energy is provided from the electrical system of the vehicle. This system includes the ignition circuit which produces sharp pulses and this may result in pulses superimposed on the direct current potential. These pulses will oppose each other in the two winding sections 21a and 21b and tend to balance each other out. This reduces the effect of the pulses and is obviously a desired condition.

It is desirable in many applications that audio output stages be suitable for connection of more than one loudspeaker thereto. For example, in an automobile radio receiver as previously mentioned, although it is desired to provide a receiver which is as inexpensive as possible, it is preferable that the receiver be easily converted for use with two loudspeakers, one of which may be provided at the front and the other at the rear of the automobile. The circuit of FIG. 1 may be easily modified for connection of two loudspeakers, and arrangements for doing this are shown in FIGS. 2 and 3.

The circuit of the transistor stage up to the terminals 22, 23 and 24 is the same in FIGS. 2 and 3 as in FIG. 1. However, in order to provide a plurality of loudspeakers the center tapped loudspeaker winding is disconnected from the terminals 22, 23 and 24. In FIG. 2 a transformer 3% has its primary winding connected between the terminals 22 and 24. A resistor 31 is connected between terminals 23 and 24. In the event that the pilot light is not used in the circuit as shown in FIG. 1, the resistor 31 is not necessary. However, if the pilot light is so connected, a small resistor can be connected between terminals 23 and 24 so that the pilot light remains operative.

The transformer 30 has its secondary winding connected to an output circuit including potentiometer 32 and windings 33 and 34 of two loudspeakers. In the event that the circuit of FIG. 1, having a single loudspeaker, is modified for use of two loudspeakers, the loudspeaker 20 of FIG. 1 may be used and the winding 21 thereof may be connected as one of the windings 33 and 3d. The common connection between windings 33 and 34 is connected to the movable contact of potentiometer 32. Accordingly, by adjustment of the movable Contact of potentiometer 32, audio signals can be supplied to the loudspeaker windings 33 and 34 in any desired proportion so that the relative audio outputs of the two speakers is thereby controlled.

The transformer 33 isolates the loudspeakers from the direct current path through the transistor so there is no direct current through the loudspeaker windings. Accordingly, the speaker diaphragm is not biased away from its normal position. The transformer may be selected to provide optimum impedance matching to the transistor and there is no loss of audio power so that maximum signals are applied to the two loudspeakers.

The circuit of FIG. 3 is generally like the circuit of FIG. 2 except that a choke 35 is provided between terminals 22 and 24 instead of the transformer 30. Connected across the choke is the potentiometer 32 and the speaker windings 33 and 34 as in FIG. 2. The movable contact of potentiometer 32 may be adjusted to control the relative levels of the signals applied to the windings 33 and 34 of the loudspeakers. In the circuit of FIG. 3 most of the direct current through the transistor will be applied through the choke 35. However, a small amount of direct current may be applied through the windings 33 and 34 of the loudspeakers but this will not be sufficient to bias the windings to an objectionable extent. In FIG. 3 the pilot light 25 of FIGS. 1 and 2 is omitted and resistor 36 is substituted therefor. The resistor 31 shown in FIG. 2 is not required in this circuit. It will be noted that the loudspeaker 20 of FIG. 1 could be connected to the terminals 22, 23 and 24 of FIG. 3 as shown in FIG. 1.

The audio output stage in accordance with the invention is provided as a very inexpensive circuit and yet efficient and high quality operation takes place. The loudspeaker may be effectively matched with the transistor by positioning the tap on the loudspeaker winding and the balancing action is controlled independently of this. This makes it possible to use a loudspeaker winding of standard construction, with the tap positioned to provide the desired impedance matching. The circuit is readily adaptable for the connection of a plurality of speakers and when modified for such use provides optimum output as is desired when an additional speaker is used.

I claim:

1. An audio system including in combination, an audio output stage including a transistor having output electrodes, a loudspeaker having a winding with end terminals and an intermediate terminal defining first and second winding sections, potential supply means for providing a direct current potential, means connecting said intermediate terminal of said loudspeaker winding to a reference potential, an output circuit including said output electrodes of said transistor connected from said potential supply means to one end terminal of said loudspeaker winding whereby direct current flows through said first winding section to the reference potential, and a second circuit including resistor means forming the sole connection from said potential supply means to the other end terminal of said loudspeaker winding, said resistor means controlling the current in said second circuit so that the current in said second winding section produces a magnetic field which cancels the magnetic field produced by the current from said potential supply means in said first winding section.

2. An audio system including in combination, an audio output stage including a transistor having output electrodes, a loudspeaker having a winding with end terminals and an intermediate terminal defining first and second winding sections, potential supply means for providing a direct current potential, means connecting said intermediate terminal of said loudspeaker winding to a reference potential, an output circuit including said output electrodes of said transistor connected from said potential supply means to one end terminal of said loudspeaker Winding whereby direct current flows through said first winding section to the reference potential, and a circuit including a pilot lamp forming the sole connection from said potential supply means to the other end terminal of said loudspeaker winding, said pilot lamp having a resistance such that the current in said second circuit through said second winding section produces a magnetic field which substantially cancels the magnetic field produced by the current from said potential supply means in said first winding section.

3. In an audio system having first, second and third terminals for selective connection to the end terminals and an intermediate terminal respectively of a loudspeaker winding, and to the terminals of inductor means and resistor means connected in series for energizing loudspeaker means connected to the inductor means, the combination including, a transistor having base, emitter and collector electrodes, direct current potential supply means, circuit means connected to said base and emitter electrodes providing input signals thereto, said circuit means being connected to said potential supply means and providing bias potentials to said base and emitter electrodes, means connecting said third terminal to a reference potential, means connecting said collector electrode to said first terminal whereby output signals and direct current are developed between said first and third terminals, and a circuit including a pilot lamp connecting said potential supply means to said second terminal to apply a direct current potential between said second and third terminals.

4. An audio system including in combination, an audio output stage including a transistor having base, emitter and collector electrodes, circuit means connected to said base and emitter electrodes providing input signals and bias potential thereto, a loudspeaker having a tapped Winding including first and second winding sections, means connecting the tap on said loudspeaker winding to a reference potential, means connecting said collector electrode to said first winding section whereby output signals and direct current flow through said first winding section, and passive resistance means connected to said second winding section and forming the sole connection thereto to apply direct current thereto so that the current in said second winding section produces a magnetic field which opposes the magnetic field produced by the current in said first winding section.

5. An audio system including in combination, an audio output stage including a transistor having base, emitter and collector electrodes, direct current potential supply means, circuit means connected to said base and emitter electrodes providing input signals thereto, said circuit means being connected to said potential supply means and providing bias potentials to said base and emitter electrodes, a loudspeaker having winding with end terminals and an intermediate terminal defining first and second Winding sections, means connecting said intermediate terminal of said loudspeaker Winding to a reference potential, means connecting said collector electrode to one of said end terminals of said winding whereby output signals and direct current flow through said first winding section, and a second circuit connecting the other one of said terminals of said winding to said potential supply means to apply direct current thereto, said second circuit including only passive resistance means having a value such that the current in said second winding section produces a magnetic field which substantially cancels the magnetic field produced by the current in said first winding section.

6. An audio system in accordance with claim 5 wherein said second circuit includes a pilot lamp for indicating when said system is energized.

References Cited in the file of this patent UNITED STATES PATENTS 2,835,735 Moen May 20, 1958 2,959,640 Schultz Nov. 8, 1960 FOREIGN PATENTS 325,021 Great Britain Feb. 13, 1930 

4. AN AUDIO SYSTEM INCLUDING IN COMBINATION, AN AUDIO OUTPUT STAGE INCLUDING A TRANSISTOR HAVING BASE, EMITTER AND COLLECTOR ELECTRODES, CIRCUIT MEANS CONNECTED TO SAID BASE AND EMITTER ELECTRODES PROVIDING INPUT SIGNALS AND BIAS POTENTIAL THERETO, A LOUDSPEAKER HAVING A TAPPED WINDING INCLUDING FIRST AND SECOND WINDING SECTIONS, MEANS CONNECTING THE TAP ON SAID LOUDSPEAKER WINDING TO A REFERENCE POTENTIAL, MEANS CONNECTING SAID COLLECTOR ELECTRODE TO SAID FIRST WINDING SECTION WHEREBY OUTPUT SIGNALS AND DIRECT CURRENT FLOW THROUGH SAID FIRST WINDING SECTION, AND PASSIVE RESISTANCE MEANS CONNECTED TO SAID SECOND WINDING SECTION AND FORMING THE SOLE CONNECTION THERETO TO APPLY DIRECT CURRENT THERETO SO THAT THE CURRENT IN SAID SECOND WINDING SECTION PRODUCES A MAGNETIC FIELD WHICH OPPOSES THE MAGNETIC FIELD PRODUCED BY THE CURRENT IN SAID FIRST WINDING SECTION. 